The invention relates to an X-ray diagnostic generator exhibiting a fixed or switchable no-load output voltage of the high voltage transformer, and comprising a control circuit for the X-ray tube voltage which contains a comparator member for comparing the actual value of the X-ray tube voltage with a nominal value, as well as a control element for the heating current of the X-ray tube for the purpose of achieving conformity of the actual value of the X-ray tube voltage with the nominal value.
An X-ray diagnostic generator of this type is described in the U.S. Pat. No. 3,974,387. In the case of an X-ray diagnostic generator such as this, the voltage drop at the internal resistance of the X-ray generator is adjusted such that the respectively desired X-ray tube voltage is connected to the X-ray tube. If the actual value of the X-ray tube voltage, in comparison with the nominal value, for example, is too high, an increase in the X-ray tube current results. Consequently, the voltage drop at the internal resistance of the X-ray generator increases and the X-ray tube voltage decreases. In the inverse case, an increase in the X-ray tube voltage results through a reduction of the X-ray tube current.
It is known that an optimum utilization of the load carrying capacity of the X-ray tube, and hence a very short photographic exposure time for X-ray photographs can be achieved if an X-ray diagnostic generator is operated with a decreasing (or "falling") load; i.e., if the X-ray tube power output is exponentially decreased from a peak value at the commencement of an X-ray photograph to a constant value. To this end, it is known e.g. from the German Offenlegungsschrift 2,122,138 to exponentially decrease the X-ray tube current from a peak value at the commencement of an X-ray photograph corresponding to the course of the maximally permissible X-ray tube power. However, this principle is not applicable in the case of an X-ray diagnostic generator of the type initially cited, since the X-ray tube current is, of course, utilized for the purpose of regulating the X-ray tube voltage.